The present disclosure relates to the field of fluid flow measurement and control and, more particularly, to a pressure-based mass flow controller system for accurately controlling the delivery of low pressure vapors from a plurality of precursors.
In the semiconductor manufacturing industry, it is necessary to achieve precise control of the quantity, temperature and pressure of one or more reactant materials which are delivered in the gaseous state to a reaction chamber. Some process reactants, such as nitrogen gas, are relatively easy to deliver in a controlled manner at the temperatures and pressures required for the reaction to occur. Other reactants, however, may be highly corrosive, toxic, pyrophoric, or unstable at the temperatures and/or pressures at which delivery to the reaction chamber is required. Such characteristics of the reactants make their accurate and controlled delivery to a reaction chamber extremely difficult to achieve.
Mass flow controllers (hereinafter, xe2x80x9cMFCsxe2x80x9d) are widely used in the industry to control the delivery of process reactants. Two broad categories of MFCs, thermal and pressure-based, have been developed to handle the diverse delivery requirements of a wide variety of process reactants. Thermal mass flow controllers operate on the principle that the rate of heat transfer from the walls of a flow channel to a fluid flowing in laminar flow within the channel is a function of the difference in temperatures of the fluid and the channel walls, the specific heat of the fluid, and the mass flow rate of the fluid. Thus, the rate of mass flow of a fluid (in the laminar flow regime) can be determined if the properties of the fluid and the temperatures of the fluid and tube are known.
On the other hand, pressure-based MFCs operate on the principle that changes in fluid pressure induce deflections in a deformable electrode, the deflections causing corresponding changes in the electrical capacitance of the deformable electrode and a stationary one coupled therewith. Pressure-based MFCs, which include, for example, capacitance manometer pressure transducers, are capable of controllably delivering process reactants at inlet pressures of less than 1 torr to greater than atmospheric pressure (760 torr).
Distinct flow regimes of a flowing fluid are recognized and defined by different pressure profiles within the fluid. Molecular flow occurs at fluid pressures of less than about 1 torr, and the flow rate of a fluid through a flow restrictive device, such as a nozzle, in the molecular flow regime is proportional to the pressure drop across the flow restrictive device. Laminar flow occurs at fluid pressures of greater than about 10 torr, and the flow rate of a fluid through a flow restrictive device in the laminar flow regime is proportional to the difference of the squares of the upstream and downstream pressures.
The pressure-based mass flow controllers disclosed in, for example, U.S. Pat. No. 3,851,526 to Drexel and U.S. Pat. No. 5,445,035 to Delajoud operate on the assumption that the fluid flow remains laminar. This assumption of laminar fluid flow limits the utility of these pressure-based MFCs to laminar flow conditions and leads to inaccuracies when such MFCs are used to characterize non-laminar flows.
In another pressure-based mass flow controller, exemplified by the Model 1150 mass flow controller manufactured and sold by the assignee of the present disclosure, the necessity for assuming laminar flow is avoided by creation of a viscous choked flow condition in the system. To establish viscous choked flow, two pressure reservoirs are created along the flow path of the fluid, for example, by introducing a restriction in the diameter of the flow path using means for defining a flow restrictive aperture, such as an orifice or nozzle. In the reservoir upstream of the flow restrictive aperture the fluid has a pressure P1 and a density xcfx811, and in the reservoir downstream of the flow restrictive aperture the fluid has a pressure P2 and a density xcfx812.
As can be seen in the graph of FIG. 1, the relationship between mass flow of a fluid and the fluid pressure upstream of a flow restrictive device is linear above a certain critical pressure and nonlinear below that critical pressure. More specifically, when the upstream pressure P1 is at least twice as great as the downstream pressure P2 (i.e., P1/P2xe2x89xa72,) the flow is said to be choked, and the flow rate is a function only of P1xcfx811 and the cross-sectional area A of the flow restrictive aperture. In general, choked flow is typically established by maintaining the upstream fluid supply at a pressure that is always at least about twice that of the fluid in the downstream processing chamber. In a choked flow regime, as the pressure of the fluid in the upstream reservoir increases, the density and flow rate of the fluid also increase.
As shown in the graph of FIG. 1, this relationship between flow rate and upstream pressure is linear so long as the upstream pressure remains at least twice that of the downstream pressure. However, when the upstream pressure is less than twice the downstream pressure (i.e., P1/P2 less than 2,), the flow is said to be unchoked and the relationship between mass flow rate and downstream fluid pressure is nonlinear.
The pressure at which a precursor fluid (typically a gas) in the upstream reservoir of a choked flow system is maintained is, in part, a function of the vapor pressure of the precursor (liquid or solid) from which the gas is derived and the desired quantity of precursor to be delivered. Some precursors, typically liquids, used in vapor deposition processes have vapor pressures which are sufficiently high to ensure their delivery at a pressure which establishes choked flow and thus allows accurate measurement of mass flow. Other precursors, particularly low vapor pressure liquids and non-dissolved solids which must be sublimed to provide reactants in gaseous form, typically cannot be delivered at a sufficiently high pressure to ensure choked flow. As a result, the mass flow rate of such precursors cannot be accurately or reliably determined.
No special provision was made to permit a pressure-based MFC, calibrated for choked flow operation, to operate in the non-linear, non-choked flow region. In the Model 1150 mass flow controller, for example, only the upstream fluid pressure is measured, although computer modeling is used to predict the mass flow in the non-linear range. In such choked-flow devices, any measurements of flow rate are assumed to be linearly related to the upstream pressure, as seen by the dotted line A in FIG. 1, even though the upstream pressure is actually less than twice the downstream pressure. In the non-choked flow regime, i.e., when the upstream pressure is less than twice the downstream pressure, the flow rate of the fluid varies as a function of the downstream fluid pressure and is independent of the upstream fluid pressure.
In a more recent pressure-based mass flow controller, exemplified by the Model 1153 mass flow controller manufactured and sold by the assignee of the present disclosure, the necessity for assuming the flow rate is linearly related to the upstream pressure, even in the non-choked flow regime, is avoided. The controller includes a flow restrictive element in the precursor flow path, and the pressures of the fluid upstream and downstream of the flow restrictive element are measured. The ratio of the upstream and downstream fluid pressures is computed to determine whether the flow is choked or non-choked. The mass flow of the precursor fluid is then computed by a CPU in accordance with a linear function of the upstream pressure, for choked flow, and in accordance with a nonlinear function of both the upstream and downstream pressures, for non-choked flow. Frequent re-calibration of the mass flow controller is not needed.
What is desired now is mass flow controller system which is suitable for use in the delivery of many types of precursor materials over a relatively wide range of operating temperatures, pressures and flow rates, without the need for frequent re-calibration.
The present disclosure provides a pressure based mass flow controller for controlling the flow rate of a vapor from a source. Such a controller can be used, for example, in the semiconductor manufacturing industry to precisely deliver a process vapor to a process chamber for making a semiconductor wafer. The disclosed flow controller can be used with a low vapor pressure source, and has a simplified, novel design, that allows the disclosed flow controller to be easily and inexpensively incorporated in a system including a plurality of mass flow controllers and a plurality of vapor sources.
The flow controller includes a flow path for connection to a vapor source, a flow restrictor dividing the flow path into an upstream reservoir and a downstream reservoir, an upstream pressure measurement device connected to the upstream reservoir, and a flow valve connected to the flow path before the upstream reservoir.
The flow controller also includes a control device programmed to receive a predetermined desired flow rate, receive an indication of upstream pressure from the upstream pressure measurement device, and receive an indication of downstream pressure from a remote downstream pressure measurement device connected to the downstream reservoir. The control device is also programmed to determine an actual mass flow rate of vapor through the flow path during choked flow conditions in accordance with a linear function of the upstream pressure, and determine the actual mass flow rate of the vapor through the flow path during non-choked flow conditions in accordance with a nonlinear function of the upstream pressure and the downstream pressure.
Choked flow conditions exist when the upstream pressure is at least equal to about twice the downstream pressure, and non-choked flow conditions exist when the upstream pressure is less than about twice the downstream pressure. The control device is also programmed to instruct the flow valve to increase flow if the actual flow rate is less than the desired flow rate, and to decrease flow if the actual flow rate is greater than the desired flow rate.
The present disclosure also provides a system for controlling the flow rates of vapors derived from a plurality of sources. The system includes a mass flow controller as described above for each of the plurality of sources. The system further includes a manifold connecting the downstream reservoirs of the mass flow controllers, and a downstream pressure measurement device connected to the manifold for providing an indication of the downstream pressure to the control devices of the flow controllers.
Another system for controlling the flow rates of vapors derived from a plurality of sources is also provided. The system includes a flow path for connection to each of the plurality of vapor sources, flow restrictors dividing each of the flow paths into an upstream reservoir and a downstream reservoir, and upstream pressure measurement devices connected to each of the upstream reservoirs. A manifold connects the downstream reservoirs of the mass flow controllers, and a downstream pressure measurement device connected to the manifold.
The system also includes a control device programmed to, for each flow path, receive an indication of upstream pressure from the upstream pressure measurement device of the flow path, and receive an indication of downstream pressure from the downstream pressure measurement device. The control device then determines an actual mass flow rate of vapor through the flow path in accordance with a linear function of the upstream pressure during choked flow conditions, and determines the actual mass flow rate of the vapor through the flow path in accordance with a nonlinear function of the upstream pressure and the downstream pressure during choked flow conditions.